What is the precision on timers?

We have built and run the code in examples->peripherals->timer in nRF5_SDK_12.3.0_d7731ad on 4 different Nordic DK nRF51 development boards. The timer period is 5 msec. We commented out the LED initialization and toggling. In the ISR, we toggle a GPIO pin and monitor the output on a logic analyzer. We find that the timer period varies from board to board, ranging from 4994 to 5025 msec. We replaced the nrf_drv_timer_xxx functions with direct programming of registers and get identical results. This is a problem because our nRF51 product needs a very accurate 5 msec "clock", and we don't want to have to hand calibrate the timer initialization with varying values for each board produced.

Can you confirm this problem and perhaps suggest a solution?

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  • Although adding NRF_CLOCK->TASKS_HFCLKSTART = 1 improved the accuracy of the timer in the Nordic sample app (without softdevice), it caused our app -- which uses soft device S130 -- to fail. We embedded exactly the same timer code (using nrf_drv_timer_xxx calls), and they work OK, except we find again that the timer is again slightly off from 5000 microseconds, depending upon the board we're using.

    We poked around and eventually added

    #include "nrf_drv_clock.h"

    and then

    	nrf_drv_clock_init();
    	nrf_drv_clock_hfclk_request(NULL);
    

    before calling app_timer_create() for the one-second timer we use in the app.

    That worked! The timer is now correct.

    Thanks in advance for your help.

  • You can also try my timer driver here. Precision in usec even with low freq timer. Driver code in src. example in exemples folder.

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